Augmented Reality

A quick overview of AR concepts

As AR has become increasingly popular in the media over the last few years, unfortunately, several distorted notions of Augmented Reality have evolved. Anything that is somehow related to the real world and involves some computing, such as standing in front of a shop and watching 3D models wear the latest fashions, has become AR. Augmented Reality emerged from research labs a few decades ago and different definitions of AR have been produced. As more and more research fields (for example, computer vision, computer graphics, human-computer interaction, medicine, humanities, and art) have investigated AR as a technology, application, or concept, multiple overlapping definitions now exist for AR. Rather than providing you with an exhaustive list of definitions, we will present some major concepts present in any AR application.

Sensory augmentation

The term Augmented Reality itself contains the notion of reality. Augmenting generally refers to the aspect of influencing one of your human sensory systems, such as vision or hearing, with additional information. This information is generally defined as digital or virtual and will be produced by a computer. The technology currently uses displays to overlay and merge the physical information with the digital information. To augment your hearing, modified headphones or earphones equipped with microphones are able to mix sound from your surroundings in realtime with sound generated by your computer.

Displays

The TV screen at home is the ideal device to perceive virtual content, streamed from broadcasts or played from your DVD. Unfortunately, most common TV screens are not able to capture the real world and augment it. An Augmented Reality display needs to simultaneously show the real and virtual worlds.

One of the first display technologies for AR was produced by Ivan Sutherlandin 1964 (named "The Sword of Damocles"). The system was rigidly mounted on the ceiling and used some CRT screens and a transparent display to be able to create the sensation of visually merging the real and virtual.

Since then, we have seen different trends in AR display, going from static to wearable and handheld displays. One of the major trends is the usage of optical see-through (OST) technology. The idea is to still see the real world through a semitransparent screen and project some virtual content on the screen. The merging of the real and virtual worlds does not happen on the computer screen, but directly on the retina of your eye, as depicted in the following figure:

The other major trend in AR display is what we call video see-through (VST) technology. You can imagine perceiving the world not directly, but through a video on a monitor. The video image is mixed with some virtual content (as you will see in a movie) and sent back to some standard display, such as your desktop screen, your mobile phone, or the upcoming generation of head-mounted displays as shown in the following figure:

In this book, we will work on Android-driven mobile phones and, therefore, discuss only VST systems; the video camera used will be the one on the back of your phone.

Registration in 3D

With a display (OST or VST) in your hands, you are already able to superimpose things from your real world, as you will see in TV advertisements with text banners at the bottom of the screen. However, any virtual content (such as text or images will remain fixed in its position on the screen. The superposition being really static, your AR display will act as a head-up display (HUD), but won't really be an AR as shown in the following figure:

Google Glass is an example of an HUD. While it uses a semitransparent screen like an OST, the digital content remains in a static position.

AR needs to know more about real and virtual content. It needs to know where things are in space (registration) and follow where they are moving (tracking).

Registration is basically the idea of aligning virtual and real content in the same space. If you are into movies or sports, you will notice that 2D or 3D graphics are superimposed onto scenes of the physical world quite often. In ice hockey, the puck is often highlighted with a colored trail. In movies such as Walt Disney'sTRON (1982 version), the real and virtual elements are seamlessly blended. However, AR differs from those effects as it is based on all of the following aspects (proposed by Ronald T. Azumain 1997):

It's in 3D: In the olden days, some of the movies were edited manually to merge virtual visual effects with real content. A well-known example is Star Wars, where all the lightsaber effects have been painted by hand by hundreds of artists and, thus, frame by frame. Nowadays, more complex techniques support merging digital 3D content (such as characters or cars) with the video image (and is called match moving). AR is inherently always doing that in a 3D space.

The registration happens in real time: In a movie, everything is prerecorded and generated in a studio; you just play the media. In AR, everything is in real time, so your application needs to merge, at each instance, reality and virtuality.

It's interactive: In a movie, you only look passively at the scene from where it has been shot. In AR, you can actively move around, forward, and backward and turn your AR display—you will still see an alignment between both worlds.

Interaction with the environment

Building a rich AR application needs interaction between environments; otherwise you end up with pretty, 3D graphics that can turn boring quite fast. AR interaction refers to selecting and manipulating digital and physical objects and navigating in the augmented scene. Rich AR applications allow you to use objects which can be on your table, to move some virtual characters, use your hands to select some floating virtual objects while walking on the street, or speak to a virtual agent appearing on your watch to arrange a meeting later in the day. We will look at how some of the standard mobile interaction techniques can also be applied to AR. We will also dig into specific techniques involving the manipulation of the real world.

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